A power chuck, such as described in my copending application Ser. Nos. 40,126 (now abandoned); 40,127 (now U.S. Pat. No. 4,243,236); 40,128 (now U.S. Pat. No. 4,243,237); 40,271; 40,272; and 40,433 (now abandoned), all filed May 18, 1979, has a chuck body which is mounted on a lathe spindle and is rotated thereby. Radially displaceable jaws on the chuck body are all coupled to an actuating member which is displaceable axially relative to the chuck body to radially displace the jaws thereof into or out of engagement with a workpiece. Typically the lathe spindle as well as the chuck are tubular so that rod stock can be fed axially through the spindle to the chuck for serial production of like workpieces, in particular when the lathe is operated as a screw machine.
A standard actuator, such as described in U.S. Pat. No. 2,835,227 or the corresponding German patent publication No. 1,018,696 filed 8 March 1954 by L. Gamet, has an actuator body that is fixed to the chuck body, a piston that is axially reciprocal within the actuator body and which is connected to the actuating element of the chuck, and a connector body which is rotatable on the actuator body and carries hydraulic connections for pressurizing the compartments in the actuator body that axially flank the piston. In this arrangement the piston is formed as a radially projecting flange on a sleeve that is axially slidable within the actuator body, and that in turn is fixed to another sleeve that extends axially out of the actuator for connection to the actuating element of the chuck.
The connector body is mounted via roller bearing on the actuator body with extremely small clearance in at least one location. At this location one of the bodies is formed with circumferential grooves open toward the other body which in turn is formed with radially extending passages aligned with these grooves. The connector body has hydraulic connections connectable via a valve to a source of fluid under pressure and a sump to pressurize the compartments and operate the chuck by the actuator.
The hydraulic fluid or oil used to pressurize the compartments flanking the piston therefore passes across a very narrow gap between the two bodies, forming a boundary layer. As these two bodies fit together very well, but without touching, this boundary layer acts as a fairly effective seal between the passages relative to each other and between the passages and the surrounding atmosphere. In fact the amount of leakage from this boundary layer can be easily controlled so that it serves to lubricate the working parts of the actuator.
The shear in this oil film, however, creates considerable heat. As the radius between the film and the axis increases, the amount of shear also increases considerably. Since such an actuator must normally form an axially throughgoing passage, limits are set to reduction of this radius, so that under the best of circumstances the radial spacing will be relatively large.
As a result there is considerable heating of this boundary layer. It is therefore common practice, as suggested in the above-cited German patent publication, to provide the actuator and/or connector bodies with cooling fins. Furthermore it is standard procedure to circulate the hydraulic fluid through a cooler before returning it to the arrangement. The first expedient has proven itself relatively ineffective in dissipating large amounts of heat, and presents dangerous radially extending vanes on which a lathe operator can be hurt. The second expedient is relatively expensive, increasing first and operating costs.